Homogeneous Ignition - Chemical Kinetic Studies for IC-Engine Applications

University dissertation from Combustion Physics, Box 118, S-221 00 Lund

Abstract: Calculations on a Homogeneous Charge Compression Ignition (HCCI) engine have been performed. Zero-dimensional models were used. The simplest model compressed the gas to auto-ignition, using temperature and pressure at a certain crank angle position obtained from engine experiments. It was found that calculations with good agreement could be accomplished, if using correct temperature, pressure and air/fuel mixture composition. However, the calculations proved to be extremely sensitive to even small variations in temperature. Further, natural gas engine calculations showed a high sensitivity to the contents of higher hydrocarbons such as ethane, propane and butanes. The validity of the kinetic mechanism was also a crucial factor. Due to the assumption of total homogeneity in the combustion chamber, a too rapid heat release was predicted. Two interfaces were developed, coupling the chemical kinetics code to existing engine simulation tools. These combined kinetics calculations and engine simulations proved to be an efficient tool for HCCI-engine analyses. Methods for mechanism reductions were developed, and implemented in the kinetic code. This was a stepwise procedure where the first part was to apply the quasi steady-state assumption (QSSA) on HCCI-calculations, where a measure of the species life-time was used to determine which species should be considered as steady-state species. The method showed a good agreement compared to the original mechanism, even for a relatively large degree of reduction. Sensitivity analysis and reaction flow analysis was combined in a semi-automatic method to generate skeletal mechanisms. The skeletal mechanisms give a good agreement, but only for a limited degree of reduction. A fully automatic method for reduction over a selected range of physical parameters was developed. It combines the two methods by applying QSSA on an automatically generated skeletal mechanism. The suggested method showed good agreement and an excellent potential for future tailor made reaction mechanisms, using a detailed reaction mechanism as a basis. A reaction mechanism for formaldehyde, methane and methanol was developed. The aim for this work was to produce a C1 mechanism of general characteristics, covering formaldehyde, methane, and possible methanol, giving correct species profiles for intermediate products. The mechanism was capable of accurately predicting ignition delays for formaldehyde and methane over a wide range, gave decent methanol auto-ignition prediction, and could further accurately predict the species profiles for formaldehyde but was not capable of calculating flame speeds for methane. A semi-detailed reaction mechanism for Primary Reference Fuels, mixtures of iso-octane and n-heptane, was developed. The predictions of ignition delay times showed a good agreement to experiments. The mechanism proved to be numerically efficient compared to mechanisms of equivalent accuracy.

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